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Lecture 8: Touch & Chapter 13: TouchTouch = A system we rarely notice needing to rely on. However, the sense of touch is very effective in
humans. This becomes particularly clear when we can’t rely on our other senses.
For example: Helen Keller > a deaf-blind women who learned to speak using only her sense of touch.
This ability allowed Helen Keller to interact with the world, and eventually complete a bachelor’s
degree, become active in politics, and lecture around America. The method seen here is called
Tadoma, but is rarely used now. Deaf-blind people instead use various forms of touch sign language,
where again touch is used to convey complex information.
HOW DOES THE BRAIN PROCESS TOUCH INFORMATION TO ACHIEVE THIS?
Touch is related to all the senses. Several types of touch are: light touch
(texture), deep touch (pressure), stretch, heat, cold and pain.
So these types of ‘touch’ often detect different physical forces in the world,
which is different from our other senses. The main thing that unites these very
different sensations is that they mainly rely on one sensory organ, the skin,
and similar processing in the brain.
The skin is the main sensory organ for touch. The skin is the largest organ in the body and is one of
the most complex sensory organ. It has more nerve fibres running to the brain than the retina does,
though the number or sensory receptors in the retina is far greater.
A cross-section through the skin
Epidermis (outer protective layer)
Dermis (hold most sensory receptors & glands)
Hypodermis (connects skin to underlying muscles)
The epidermis protects the underlying tissue from water and sunlight, but does not contain sensory
receptors. The dermis contains sensory receptors and sweat glands. The hypodermis is also called the
subcutis which contains a few sensory receptors and the subcutaneous fat which is important for
thermal insulation in warm-blooded mammals. The hypodermis connects skin to underlying muscles,
joins & bones.
SENSORY RECEPTORS
The dermis is the most important layer for sensory receptors. Sensory receptors respond to physical
forces on the skin. Mechanoreceptors respond to mechanical force and there are four types.
1)Meissner corpuscle: signals changes and stops responding to constant pressure (= adaptation rate).
Lie near the surface and so have small receptive field sizes.
2) Merkel cell complex: slow adaptation rate. Lie near the surface + have small receptive field sizes.
3) Ruffini ending: slow adaptation rate. Deep location and have large receptive field size.
4) Pacinian corpuscle: this receptor has a deep location, because it lies in the hypodermis. It has a
large receptive field. It is made up of lots of layers. This receptor uses a simple mechanism of
depolarisation. There are a few important things to note about this depolarisation. First, the pressure
must be above a certain strength to trigger an action potential (pressure B). Second, the receptor’s
membrane potent rapidly returns to its polarised state when the pressure is left on. So this receptor
will rapidly adapt to a new pressure state. Third, if the pressure is greater (pressure C) the membrane
potential remains above the action potential trigger threshold for longer, so makes more action
potentials.
, > So this receptor is only sensitive to changes in pressure and the size of the change is encoded by
the number of action potentials. Deep location and have large receptive field size.
All touch receptors have some extended shape, where pressure on the skin leads to a change in the
shape of the receptor. This change in the receptors shape leads to some change in membrane
potential.
BUT WHY DO WE NEED FOUR OR MORE DIFFERENT RECEPTORS FOR TOUCH?
There is a difference in the adaptation rate: does the receptor signal
pressure or changes in pressure? The Pacinian corpuscle signals
changes and stops responding to constant pressure. The Meissner
corpuscle also does this. The Merkel cell complex and Ruffini ending
keep responding if the pressure stays, and decrease their response
only slowly: they have a slow adaptation rate.
Besides that, there is a difference in the receptive field size: does the
receptor respond to touch over a large area or a small area? The
Pacinian corpuscles and Ruffini endings are deep in the skin, so
sensitive to pressure over a large area. Meissner corpuscles and
Merkell cell complexes are much closer to the surface, so only
respond to pressure in a small, local area, a small receptive field.
Meissner corpuscles = small receptive field + fast adaptation
Merkel cell complex = small receptive field + slow adaptation
Pacinian corpuscle = large receptive field + fast adaptation
Ruffini ending = large receptive field + slow adaptation
Meissner corpuscles Merkel cell complex Pacinian corpuscle Ruffini ending
Receptive field Small Small Large Large
Adaptation Fast Slow Fast Slow
Two point detection threshold = How far apart must two points lie on the skin for us to reliably
distinguish between one point and two. Where this threshold is low, that means points can be
distinguished close together, and the skin is sensitive here. This is what we find on the hands, face
and feet. The rest of the body, the threshold is higher and we are less sensitive to fine touch details.
Suppressive surround organisation = where two stimuli are close together, the receptors involved
inhibit each other’s responses, so the stimuli feel weaker. An example of two-point threshold is the
Braille system.
TOUCH RECEPTORS IN THE MUSCLES = PROPRIOCEPTION (where our
body is in space)
These are particularly important for controlling our movements. The
simplest interaction between touch and movement is the reflex ark:
a sensation from the body immediately causes a reflexive
movement that attempts to interact with that sensation. For
example, when we touch something very hot, we reflexively draw
the hand away. Our nervous system is wired to do it automatically.
The example here is the patellar reflex. Tapping the tendon
stretches the quadriceps suddenly. The sensory fibre in the muscle
,directly activates a motor neuron that tightens the muscle to counter the stretch, kicking the leg out.
Looking closer, we see that the sensory nerve fibres and motor nerves come into the muscle together
ANSWERING QUESTIONS ABOUT TOUCH
Can we control movement without sensory feedback? (Video 1) What observation supports
this? Why is this important to understand (touch & movement)? → No, controlled
movements are difficult then.
Is this limited to somatosensory feedback? (Video 1) → No, he managed to find his way out
so visual feedback is necessary and important so you can find your way around in space.
Are touch and movement carried together to the muscles? (Video 2) Yes, the neurons that
respond to movement also carry touch neurons. Sensory and motor neurons are different
from each other.
Multisensory interactions have different types. An example is the rubber hand illusion; The subject
know that it is not their hand, but gets the impression of ownership. This shows how our sense of self
depends strongly on patterns of sensory input (seeing and feeling the touch together) that normally
only arise from our own body. Another example is an out-of-body experiences through an VR
experience in a lab. Still, we move to react to movements of the virtual body just like it was our own.
> Our sense of self depends very strongly on patterns of sensory input; seeing and feeling touched
together.
TWO TYPES OF THERMORECEPTORS
The thermoreceptors are divided in one for hot and one for cold. At body temperature, 37 degrees,
both fire evenly. If we touch an object colder than the skin, the temperature of the skin drops and
cold fibres fire more. If we touch something warmer than the skin, this increases the skin’s
temperature and the warmth nerve fibres fire more. Note this is NOT the temperature of the object,
but the temperature of the receptor, inside the body. This is always pushed towards 37 degrees by
the body. Both drop their response when very far away. If the skin reaches these temperature, this is
well outside the normal range we experience and very dangerous. Here, we feel pain instead of
heat/cold.
PAIN RECEPTORS / NOCICEPTORS
A nociceptor starts firing around 42 degrees skin temperature. The thermoreceptors are firing
maximally at around 42 degrees also. So the mixture of responses of both of these
receptors tells us there is damage to the body, or pain, and this results from heat.
Many nociceptors are located just below the skin. > There are two types:
A-delta fibres, which are myelinated for fast conduction, carry the initial response
to damage, a sharp painful sensation.
C fibres are unmyelinated, so conduct slow signals. These are less sensitive, so
only react to stronger damage. Activating them gives a feeling of more intense,
deeper pain.
These two types of nociceptors can be found in the periosteum, joints, arterial wall, liver capsule and
sparse pain never endings. Damage to any of these can strongly reduce our chances of survival.
The nociceptors are located throughout the whole body.
TO CONCLUDE
Types of touch and touch receptors: 7 mechanoreceptors types touch, pressure etc.. They are
distinguished by receptive field size and adaptation rate.
3 kinaesthetic receptor types, like muscle stretch.
2 thermoreceptors hot and cold.
4 nociceptor types, pain receptors.
, PATHWAYS TO THE BRAIN
1) Dorsal column medial lemniscal pathway: goes through the spinal cord, along the dorsal column.
This reaches the brain stem, and in the medulla crosses over to the other side of the body. From
here, it passes to the somatosensory cortex up the medial lemniscus. It terminates in our
somatosensory cortex. ‘Somatosensory’ means the sense of the body, the scientific term for our
general sense of touch, including pain, temperature and so on. It carries our sense of fine touch,
vibration and limb position / proprioception.
2) Anterolateral / spinothalamic system: fibres sensing pain and temperature. This also carries some
touch information, but lacks the fine, light sense of touch that we normally rely on. This pathway
crosses over (decussates) on entering spinal cord. This may allow easy development of reflexes to
move away from pain using muscles on the opposite side of the body. After crossing, this pathway
ascends to the brain up the anterior (front) spinal cord, where it runs straight to the brain.
Information from the thalamus runs to the limbic system, allowing pain to interact with emotions.
Anterolateral Dorsal column-medial lemniscal
Modalities Pain, temperature sense & Fine touch and pressure, vibration
crude touch proprioception (of arm only)
Location in spinal cord Anterolateral column Dorsal column
Level of decussation Spinal cord Medulla (brain stem)
Cortical terminations Primary and secondary Primary and secondary
Somatosensory cortex Somatosensory cortex
Posterior parietal cortex Posterior parietal cortex
THE BRAIN’S SOMATOSENSORY SYSTEM
Our sense of touch is processed in the brain’s somatosensory areas.
‘Somatosensory’ means the ‘sense of the body’. So the term
‘somatosensory’ refers to the whole state of the body, not only the
skin. Primary somatosensory cortex (S1) is located at the front of the
parietal lobe, on the post-central gyrus, the ridge immediately behind
the central sulcus. Immediately in front of the central sulcus, we find
the primary motor cortex (M1). The central sulcus separates the
parietal and frontal lobes, so M1 is at the back of the frontal lobe.
Just like in the visual system, each half of the brain’s somatosensory and motor maps processes
information from the opposite (contralateral) side of the body. As we just saw, the information
coming up to the brain crosses over in the spinal cord or brain stem. In V1, we see that the centre of
the visual field is processed by a larger area of the brain than peripheral vision. This allows us to have
finer visual processing in the centre of vision.
In the same way, the somatosensory and motor cortices have a larger area devoted to body parts
where we have sensitive touch or fine movements. These are often the same body parts: our hands,
faces and tongues are processed by large areas of the brain because they are sensitive to touch and
also perform very skilled movements. These are all fairly small body
areas. On the other hand, the upper body, the torso or trunk, is a
large body area but is processed by a small part of the brain. This is
because we have little touch sensitivity and can only perform very
simple movements with the torso. There are also places where the
somatosensory and motor cortex are quite different: the
somatosensory cortex has a fairly large representation of the
genitals, a very sensitive area. But the motor cortex does not: we
can’t perform fine movements with the genitals.
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